There are many physiologically active substances containing a condensed pyrrole ring, and in particular new methods of producing substances containing an indole ring have been actively studied from the 19th century to the present. Besides the Fischer method known for 100 years or more, methods such as Bischler method, Madelung method, Reissert method and a method of using a palladium catalyst have been developed, but have not surpass the Fischer method in general and economical aspects.
The Fischer method is a method of synthesizing a hydrazone from an aryl hydrazine and a ketone and then treating it with an acid to form indole. Because various kinds of ketones are easily available, this method is generally usable, but there are problems such as necessity for synthesis of hydrazine from an aniline derivative and occurrence of various regioisomers at the time of forming an indole ring, etc. Various attempts have been made on regioselective synthesis, but have never provided complete solution (J. Org. Chem., 56, 3001(1991); J. Org. Chem., 58, 7638 (1993), etc.).
The reaction scheme of synthesizing indole by the Fischer method is shown below: 
On the other hand, as organometallic chemistry has advanced for about 30 years, methods of using a palladium catalyst have been extensively developed. Such methods utilize the cross-coupling reaction unique to the palladium catalyst. These methods involve reacting o-iodoaniline with an alkyne etc. to synthesize o-alkynyl aniline and then adding an amine to the alkyne in the molecule to form an indole ring, and there are a vast number of reports thereon. The disadvantages of such methods are that o-iodoaniline is considerably more expensive than aniline; although the atomic weight of iodine is 127 which is higher than that (92) of the aniline nucleus, the iodine does not remain in the indole skeleton of the product and is discarded as a byproduct, that is, the atom efficiency is low; nitrogen in the aniline should previously been converted into e.g. amide (Chem. Pharm. Bull., 1305 (1988); Tetrahedron Lett., 3915 (1992), etc.).
The reaction scheme of synthesizing indole by the method of using a palladium catalyst is shown below: 
Further, as a method of using a ruthenium catalyst, a method of synthesizing indole from aniline and 1,2-diol is known. This method is superior in features such as atom efficiency and usability of aniline itself as a starting material, but there are problems such as formation of a mixture of isomers from asymmetric diol due to generally low regioselectivity, necessity for relatively high temperature (180° C.), necessity for an argon atmosphere, necessity for a solvent, etc. (J. Org. Chem., 52, 1673 (1987), etc.).
The reaction scheme of synthesizing indole by the method of using a ruthenium catalyst is shown below: 